Nitric acid modification of nanoporous material from long-flame coal activated with potassium hydroxide
DOI:
https://doi.org/10.15407/hftp17.03.397Keywords:
nanoporous carbon, nitric acid modification, acid groups, supramolecular structure, porosity reorganizationAbstract
The aim of the work is to evaluate the effect of low-temperature nitric acid modification on the structure of nanoporous carbon (NC) obtained by alkaline activation of long-flame coal at a low KOH/coal ratio of 1 g/g.
The initial NC is an activated (800 °C, 1 h) carbon with a pore volume of 1.074 cm3/g and a specific surface area SBET = 1729 m2/g. The NC modification was performed with HNO3 solutions (5–60 %) at 22±1 °C (24 h) and a constant HNO3/NC ratio (1 mol/mol). The NC elemental composition was determined by a Carlo Erba 1106 analyzer. The content of acidic groups (AG) was determined by the Boehm method. The oxygen content of non-acidic groups (NAG) was calculated by the difference between the AG oxygen content and the total oxygen content. The NC supramolecular structure was studied by X-ray diffraction (Bruker D8 diffractometer). The volumes and specific surface areas of ultramicropores (? 0.7 nm), supermicropores (0.7–2.0 nm), subnanopores (? 1.0 nm) and micropores (? 2.0 nm) were calculated by the 2D-NLDFT-НS method (SAIEUS program) using low-temperature nitrogen adsorption-desorption isotherms.
The concentration of [HNO3] was found to determine the composition of oxygen groups and the degree of reformation of the NC supramolecular and porous structure. With [HNO3] increase in the range of 5–60 %, the NC weight increases linearly to 7.8±1.4 %. The oxygen content of AG increases by 3.8 times (from 1.54 to 5.89 %), and the oxygen content of NAG – by 1.2 times (from 11.06 to 13.19 %). The increase in [HNO3] was found to cause a linear increase in the total content of AG (R2 = 0.952) and the content of carboxyl groups (R2 = 0.930). The content of lactone groups increases against the background of a decrease in the content of phenolic groups due to the participation of phenols in the formation of lactones. The consumption of carboxyl groups for this is not observed, because it is compensated by their formation during the NC oxidation. Low-concentration (? 20 %) HNO3 solutions were found to cause an increase in the oxygen content of AG higher than the increase in the total oxygen content, which indicates the transformation of NAG into AG. The NC supramolecular structure changes only when modified with concentrated (? 30 %) HNO3. The interlayer distance and the height of crystallites increase due to the intercalation of nitrate anions, but polyarenes do not exfoliate. The average diameter of polyarenes decreases significantly, which is due to their “burning” due to the intermediate formation of oxygen groups. Nitric acid modification causes a decrease in the total pore volume (by ~ 8 %), transforms a part (~ 26 %) of micropores into mesopores and increases the volume of ultramicropores. With increasing [HNO3], the SBET value linearly decreases by ~ 20 %. The specific surface of ultramicropores and subnanopores change through maxima, and their fractions reach 82.1 and 31.0 %, respectively. Modified NMs are microporous materials with a developed subnanoporous structure.
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